To control exhaust emission, catalytic conversion units are connected in exhaust systems of internal combustion engines. The typical catalytic conversion unit draws oxygen from the atmosphere and converts carbon monoxide and other undesired components, including unburned hydrocarbons, in the exhaust gas to water and carbon dioxide. In the past, the catalytic conversion units have been separate from mufflers used in the exhaust system to attenuate the accoustical energy.
One common form of catalytic conversion unit includes a ceramic monolith catalyst coated with a noble metal, and the ceramic material is a cell-like structure having a multiplicity of passages that extend through the structure. In the past, problems have been encountered in attempting to get uniform mixing of the oxygen and the exhaust gases prior to flowing the gases through the catalytic conversion unit, and a further problem has been encountered in obtaining uniform distribution of the gaseous mixture across the face of the catalyst. Accordingly, various types of baffles, deflectors, impellers, and the like have been used upstream of the catalytic conversion unit in an attempt to obtain the desired mixing of air and gases and the desired flow distribution.
A further problem that has been encountered in the past is that of overheating and possible degradation and/or failure of the catalyst. In certain exhaust gases having a high proportion of carbon monoxide the oxidation reaction results in the development of extremely high temperatures which can adversely affect or destroy the performance of the catalyst.